This invention relates in general to the manufacture of monolithic capacitors and, more specifically, to the production of metal electrodes on ceramic dielectric substrates for such capacitors.
Conventional multilayer capacitors generally consist of a number of alternate layers of conductive metal electrodes and dielectric layers, all connected in parallel so as to provide an increase in the electrical capacitance for a given area. This structure is generally referred to as a monolithic construction of electrodes and dielectrics or as a monolithic capacitor. The dielectric may be an insulating synthetic resin, a ceramic material or other insulator. A variety of conducting materials, typically metals, may be used in the electrodes.
At the present time, the electrodes are formed on a dielectric substrate through a "silk screen" printing process in which a type of printing ink comprising finely divided precious metal particles (typically having diameters of about 1 micrometer) dispersed in a resinous carrier are forced through a screen stencil onto the substrate. The conductive ink is generally deposited to a thickness of at least about 0.001 inch in order to provide the required conductivity. Because of the particulate nature of the conductor and the non-conductive resin matrix, the resulting layer does not have an optimum high conductivity. As the layer is made thinner than about 0.001 conductivity decreases to an undesirable degree, decreasing at an exponential rate until substantially zero conductivity results when the layer has a thickness substantially equal to the metal particle diameter. With 1 micrometer (0.0004 inch) particles, about 25 layers of particles are required to provide the necessary 0.001 inch thickness. Such electrodes have been found to be effective with dielectric substrates which also have thicknesses of about 0.001 inch.
Recently, dielectric materials have been developed which are effective even when used in sheets much thinner than 0.001 inch. Unfortunately, when capacitors having thick conductive electrodes on thin dielectric sheets are stacked to form multi-layer capacitors, the sheets do not conform well to the edges of the electrodes, leaving voids and sharp edges which are conducive to electrical breakdown of the capacitor in use.
Thus, the present methods of using silk screened capacitors with relatively thick electrodes and substrates tend to produce multilayer capacitors which are undesirably thick and undesirable for use with miniaturized electronic assemblies.
While silk screening remains the customary method of fabricating multilayer capacitors, attempts have been made to use other methods for applying a conductive layer to form the electrodes.
As described by Behn et al. in U.S. Pat. No. 4,376,329, simple capacitors have been made by vacuum evaporating a metal such as aluminum onto a substrate, followed by forming a layer of synthetic resin by gas polymerization, then vacuum evaporating another metal layer. This method is complex and cannot effectively produce multilayer ceramic capacitors comprising alternate layers of metal and ceramic dielectrics at high rates.
Another method of producing laminated capacitors is described by Behn in U.S. Pat. Nos. 4,378,382 and 4,508,049. Here, carriers are located in recesses in a drum, which is rotated to move the carriers alternately through vacuum chambers which deposit a metal such as aluminum, then a synthetic resin dielectric, by vacuum deposition. This is another complex system, requiring complex seals where the drum enters and leaves the vacuum chambers. This method does not seem adaptable to ceramic dielectrics and precious metal electrodes in high temperature resistant capacitors.
Thus, there is a continuing need for methods and apparatus for producing high temperature resistant monolithic capacitors using thin ceramic dielectric substrates and thin precious metal electrodes.